The present invention relates to novel dioxetane compounds, cationically polymerizable compositions containing such dioxetane compounds, optical films produced from such polymerizable compositions, and liquid crystal display devices equipped with such optical films.
In recent years, active studies and developments have been carried out so as to utilize liquid crystalline compounds as optical materials, and many of them have been already put into practical use. In order to apply a liquid crystalline compound as an optical material, it is essential that after the molecules of the compound are aligned and fixed in a liquid crystal state, the aligned state can be retained under practical use conditions. As methods for retaining a liquid crystalline compound in an aligned state, there have been proposed various methods such as those using polymerizable liquid crystalline compounds, polymeric liquid crystalline compounds, polymeric liquid crystalline compounds having crosslinkable reactive groups, and the combination of polymeric liquid crystalline compounds having crosslinkable reactive groups and polymerizable liquid crystalline compounds.
As a method using polymeric liquid crystalline compounds having crosslinkable reactive groups, there has been proposed a method using a polymerizable liquid crystalline compound having a mesogen portion comprising two or more benzene rings or similar rings, spacer portions each comprising a hydrocarbon chain, and a radically polymerizable reactive group such as a (meth)acrylate group at one or both of the terminal ends (Patent Document No. 1). In this method, such a polymerizable liquid crystalline compound is coated in a heat-melted state or in the form of a solution on an alignment substrate and dried if necessary so as to be formed into a liquid crystalline layer. Thereafter, the liquid crystalline layer thus formed is aligned in a liquid crystal state by heating and then polymerized by photo-irradiation so as to fix the layer in the liquid crystal state. However, it is necessary for this method to suppress undesired polymerization inhibition effects caused by oxygen in the air and conduct some complicated operations such as photo-irradiation under an inert gas atmosphere, requiring improvements in facilities and apparatus. Since a (meth)acrylate group is apt to polymerize with light or heat, a careful attention must be paid during the synthesis.
A liquid crystalline compound is known which contains a cationically polymerizable group in place of a (meth)acrylic group, both groups of which have an symmetrical structure viewed from the mesogen group and have characteristics that they have liquid crystallinity (Patent Document Nos. 2 and 3).
As the method using a polymeric liquid crystalline compound, it is proposed to use a liquid crystalline polyester with excellent properties to be retained in the liquid crystal orientation (Patent Document No. 4). However, as a result of the widespread of mobile devices, an optical film comprised of such a liquid crystalline polyester has been demanded to have properties to retain the liquid crystal orientation under more sever practical use conditions and more excellent mechanical strength as well.
As the method using a polymeric liquid crystalline compound having a polymerizable reactive group, there has been proposed a method wherein a polymerizable reactive group is introduced into the polymeric main chain and a method wherein a monomer unit having a polymerizable reactive group is introduced into a side chain. However, since in either of these methods, the liquid crystallinity of the material used is decreased, there is a limit to the amount of the polymerizable group to be introduced to enhance the sufficient mechanical strength. Therefore, alternative methods have been demanded (Patent Document No. 5).
As a method wherein a polymeric liquid crystalline compound having a crosslinkable reactive group is mixed with a polymerizable liquid crystalline compound, there has been proposed the use of a composition comprising a side chain type polymeric liquid crystalline compound having an oxetane group and a liquid crystalline dioxetane compound (Patent Document No. 6). This composition can be enhanced in mechanical strength by increasing the amount of the liquid crystalline dioxetane compound. However, it is necessary to add a compound that is capable of generating cations with light and/or heat in order to effect polymerization rapidly while the liquid crystal phase is retained. However, the liquid crystalline dioxetane compound is highly compatible with a liquid crystalline compound but can not be sufficient in compatibility with a non-liquid crystalline compound such as the foregoing compounds capable of generating cations because it is likely to cause phase separation when the non-liquid crystalline compound is added.
Patent Document No. 1: Japanese Patent Laid-Open Publication No. 11-080081
Patent Document No. 2: U.S. Pat. No. 5,773,178
Patent Document No. 3: Japanese Laid-Open PCT International publication No. 2004-510785
Patent Document No. 4: Japanese Patent Laid-Open Publication No. 11-158258
Patent Document No. 5: Japanese Patent Laid-Open Publication No. 9-3454
Patent Document No. 6: Japanese Patent Laid-Open Publication No. 2004-315736